1. Field of the Invention
This patent application relates to molded plastic packages for encapsulating semiconductor devices. More particularly, leadframe treatments that improve adhesion of the leadframe to a plastic molding resin are combined with specific die attach materials. The resultant packages are resistant to post-assembly fracture, such as "pop corning".
2. Background
Electronic packages to encase one or more semiconductor devices frequently have a molded polymer resin body partially encapsulating a conductive metal leadframe. The inner lead portions of the leadframe define an aperture. Frequently, a die attach paddle is disposed in this aperture. A semiconductor device is bonded to the die attach paddle and electrically interconnected to the inner lead portions by small diameter, typically on the order of 0.025 millimeter (0.001 inch), bond wires. The inner lead portions of the leadframe, the die attach paddle, the wire bonds and the semiconductor device are then encapsulated in the polymer molding resin.
One problem with the molded plastic package is that subsequent to molding, internal delamination, the separation of package components from the molding resin, frequently occurs. In severe cases, a crack develops, creating an ingress site for contaminants. Also, if moisture enters along a delamination, the package base can swell making electrical interconnection to a printed circuit board difficult or impossible.
The package has two locations particularly prone to delamination, the interface of the leadframe and the molding resin and the die attach interface between the semiconductor device and the die attach paddle.
Delamination at the interface of the leadframe and the molding resin is primarily due to inadequate adhesion between the leadframe and the molding resin. Delamination at the die attach interface is primarily due to stresses generated by coefficient of thermal expansion mismatch between the semiconductor device and the leadframe.
U.S. Pat. No. 4,888,449, to Crane et al. discloses coating copper alloy components with various metallic materials to improve adhesion to a polymer adhesive. The adhesion of a copper alloy leadframe to a polymer molding resin is improved by coating the leadframe with a mixture of chromium and zinc as disclosed in U.S. Pat. No. 5,343,073 to Parthasarathi et al. U.S. Pat. No. 5,367,196 to Mahulikar et al. discloses coating a heat spreader with an adhesion enhancing material. The Crane et al., the Parthasarathi et al. and Mahulikar et al. patents are incorporated by reference herein in their entireties.
Polymer coatings also improve the adhesion of a leadframe to a molding resin as disclosed in U.S. Pat. No. 5,122,858 to Mahulikar et al. The adhesion of a die attach paddle to a molding resin is improved by forming depressions in the die attach paddle to mechanically lock the paddle to the molding resin as disclosed in U.S. Pat. No. 4,884,124 to Mori et al.
Improving the adhesion between the leadframe and the molding resin reduces delamination. However, molded plastic packages still fail due to vulnerability at the die attach interface. Additionally, treatments which effectively improve adhesion of the leadframe to the molding resin frequently adversely affect the strength of the bond between bond wires and the inner lead portions of the leadframe.
There exists, therefore, a need for a semiconductor package which resists delamination and cracking and is characterized by high strength wire bonds.